Thursday, April 14, 2016

Autism spectrum disorder
(ASD) is a neurodevelopmental disorder characterized by impairments in (a)
social communication and social interaction and (b) restricted and/or repetitive behaviors or
interests that varies in severity of symptoms, age of onset, and
association with other disorders. ASD is both highly heritable and highly
stable across age. Although improvement in symptoms is often observed between
childhood and adolescence/ adulthood, longitudinal studies have found that
individuals diagnosed with ASD rarely move off the spectrum and that symptom
severity does not change significantly over time. Similarly, high
stability has been found when autistic traits are measured in the general population.

Research

The heritability of ASD has been established in family
and twin studies. For example, twin studies in the United Kingdom, United
States, Sweden, and Japan have established that the concordance rate for ASD in
monozygotic (MZ) twins is much higher than in dizygotic (DZ) twins, suggesting
substantial genetic influences. Likewise, twin studies of autistic traits in
community and population-based samples in middle childhood and adolescence have
found evidence for moderate to strong genetic effects, with the remaining
variance primarily explained by non-shared environmental factors

Despite extensive research into the causal influences on
autistic traits at individual ages and the finding of high stability in these
traits, little is known about the overlap in genetic and environmental
influences on autistic traits across age and the contribution of such
influences to trait stability itself. Knowing this is important for
understanding the developmental course of autistic traits and why autism
behaviors appear to vary so little over time. A research study published in the
Journal of the
American Academy of Child & Adolescent Psychiatryinvestigated
these questions in a general population sample of twins. More than 6,000 male
and female pairs of twins were rated on a scale of autistic traits by their
parents at 8, 9, and 12 years of age and by their teachers at 9 and 12 years of
age. Autistic traits were assessed using the Childhood Autism Spectrum Test
(CAST, formerly the Childhood Asperger Syndrome Test). The CAST was developed
as a screening questionnaire for use in non-clinical settings. Data were
analyzed using structural equation modeling.

Results

The results indicated
that, consistently across raters, not only were autistic traits stable and
moderately to highly heritable at individual ages, but there was also a high
degree of overlap in genetic influences across age. Autistic trait stability
was largely accounted for by genetic factors, with the environment unique to
each twin playing a minor role. The environment shared by twins had virtually
no effect on the longitudinal stability in autistic traits.

Implications

The findings of the
present study have important implications for understanding ASD. They suggest
that autistic traits are not only highly stable across middle childhood, but
that this stability is caused largely by genetic influences in both boys and
girls. This implies that stability is a biological characteristic of autistic
symptoms, as opposed to being due to shared environmental factors. A caveat! The
results do not mean that all variance in the stability of autistic traits is
caused by genetic influences, nor do they suggest that autistic traits are
fixed and resistant to change. Although the environment shared by the twins had
little influence on the stability of autistic traits, the unique environment of
each twin, such as being in different classes at school or receiving differential
parental treatment, did appear to play a modest role in the stability of
autistic traits. Despite the importance of genetics, environmental influences and
treatment/intervention could still have an effect on these characteristics.

The results of this study add
our knowledge of the key role that genes play in autistic traits and how these
behaviors manifest themselves as children grow older. Overall, the findings
suggest that the development of autistic traits in middle childhood is
characterized largely by a set of stable genetic influences, but that, in
addition to these influences, new genetic influences as well as
modest-to-moderate environmental influences emerge at each age. Future research
should include an examination of the specific nature of these causal influences
on autistic traits across the lifespan. Likewise, the findings need replication
in a clinical sample of individuals with ASD.

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